Photoacoustic spectroscopy is an analytical method that involves stimulating a sample by light and subsequently detecting sound waves emanating from the sample. Typically, only a narrow range of wavelengths of light are introduced into a sample. Such narrow range of wavelengths of light can be formed by, for example, a laser. Utilization of only a narrow range of wavelengths can enable pre-selected molecular transitions to be selectively stimulated and studied.
A photoacoustic signal can occur as follows. First, light stimulates a molecule within a sample. Such stimulation can include, for example, absorption of the light by the molecule to change an energy state of the molecule. Second, an excited state structure of the stimulated molecule rearranges. During such rearrangement, heat, light, volume changes and other forms of energy can dissipate into an environment surrounding the molecule. Such forms of energy cause expansion or contraction of materials within the environment. As the materials expand, sound waves are generated. Accordingly, an acoustic detector mounted in acoustic communication with the environment can detect changes occurring as a result of the light stimulation of the absorbing molecule.
An exemplary prior art apparatus 10 for photoacoustic spectroscopy is shown in FIG. 1. Apparatus 10 comprises a light source 12 configured to emit a beam of radiation into a sample holder 14. Light source 12 can comprise, for example, a laser. Filters (not shown) can be provided between light source 12 and sample holder 14 for attenuating the light prior to its impacting sample holder 14.
Sample holder 14 comprises a sample cell 18 containing a sample 16. Sample cell 18 can comprise a number of materials known to persons of ordinary skill in the art, and preferably comprises a material substantially transparent to the wavelengths of light emanating from light source 12. Preferred materials of sample cell 18 will accordingly vary depending on the wavelengths of light utilized in the spectroscopic apparatus. If the wavelengths of light are, for example, in the range of ultraviolet through visible, sample cell 18 can preferably comprise quartz.
Sample 16 comprises a material that substantially fills sample cell 18. Such material can be, for example, a fluid such as a liquid or a gas. Sample 16 can, for example, comprise a liquid solution wherein the molecular vibrations that are to be studied are associated with molecules dissolved in the liquid.
Apparatus 10 further comprises an acoustic detector 20 mounted to sample cell 18 and in acoustic communication with sample 16. Acoustic detector 20 can comprise a transducer, such as, for example, a microphone and can be mounted such that a fluid (for example, a grease) is provided between a surface of detector 20 and sample cell 18. Detector 20 is typically removably mounted to sample cell 18 by, for example, a clamp. Acoustic detector 20 is in electrical communication with an output device 22. Device 22 can be configured to display information obtained from detector 20, and can be further configured to process such information. Output device 22 can comprise, for example, an oscilloscope or a computer.
In operation, a beam of light is generated by source 12 and passed through sample cell 18 to stimulate molecular excitation within sample 16. Non-radioactive decay or molecular rearrangements cause expansions and/or contractions of a material within sample 16 to generate acoustic waves passing from sample 16 through sample cell 18 and to acoustic detector 20. Acoustic detector 20 then detects the acoustic waves and passes signals corresponding to, for example, amplitudes and frequencies of the acoustic waves to output device 22. Output device 22 can be configured to convert information obtained from detector 20 to, for example, a graphical display.
A difficulty in utilizing apparatus 10 is that acoustic waves emanating simultaneously within sample 16 do not reach detector 20 at the same time. As shown in FIG. 2, light from source 12 typically has a general shape of a cylinder 24 as it passes through sample cell 18. Individual acoustic waves emanating from cylinder 24 (shown as dashed lines 26) also have cylindrical shapes. All portions of an individual acoustic wave 26 are generated simultaneously within sample 16, and should therefore desirably simultaneously impact detector 20. However, as acoustic detector 20 has a flat detection surface, an individual acoustic wave 26 will impact acoustic detector 20 at a later time at an edge of the detection surface relative to a center of the detection surface. Thus, there is a spread of a time interval during which an individual acoustic wave impacts detector 20, rather than the desired simultaneous detection event. It is desirable to reduce the time interval during which an individual acoustic wave is detected to enhance sensitivity.
One approach that has been utilized for reducing such time interval is to utilize a detector 20 having a curved detection surface approximately complementary to the curved cylindrical shapes of acoustic waves 26. However, as such detectors can be difficult to make the approach has had limited success. Another approach is to use a slit to provide a planar acoustic wave.
Another approach that has been utilized for reducing a time interval during which an individual acoustic wave is detected is exemplified by a photoacoustic apparatus 10b shown in FIG. 3. In referring to the apparatus of FIG. 3, similar numbering to that utilized above in describing apparatus 10 of FIG. 1 will be used, with differences indicated by the suffix "b" or by different numerals. The primary difference between apparatus 10b and apparatus 10 of FIG. 1, is that in apparatus 10b transducer 20 is mounted directly in front of the beam of light emanating from light source 12. Accordingly, apparatus 10b comprises a sample cell 14b slightly modified from the sample cell 14 of apparatus 10 (FIG. 1). As long as transducer 20 has a detector face that is smaller in cross-sectional area than an area of the light beam emanating from source 12, individual waves generated by the light beam will reach the face at approximately the same time across an entire surface of such face. Accordingly, apparatus 10b can eliminate the above-discussed problem of individual acoustic waves reaching an acoustic detector face at a spread of time intervals across a surface of the face. A difficulty associated with apparatus 10b is that the light emanating from source 12 shines directly into a detector face of transducer 20 and can adversely heat such face. Accordingly, a shield 26 is typically provided along an internal sidewall of sample cell 18b to block radiation emanating from light source 12 from reaching a detector face of transducer 20. Shield 26 is typically a thin film, and such thin films are typically only suitable for very narrow ranges of light (about 20 nanometers on average). Accordingly, a band of light entering sample holder 18b must typically be kept to a very narrow wavelength range to avoid having light pass through film 26 and into transducer 20.
As the above discussion indicates, the apparatuses 10 and 10b of FIGS. 1 and 3, respectively, both have advantages and disadvantages. Specifically, the apparatus 10 of FIG. 1 can enable relatively large bands of light to be utilized for photoacoustic spectroscopy experiments, but has slow response times and significantly lower sensitivity due to large time intervals wherein individual acoustic waves impact different regions of an acoustic detector surface. In contrast, apparatus 10b can have rapid response times to acoustic waves generated within sample 16, but is generally only useful for relatively narrow ranges of light. It would be desirable to develop alternative photoacoustic detector systems which could accomplish the advantages of both apparatus 10 of FIG. 1 and apparatus 10b of FIG. 3.
In another aspect of the prior art, it is recognized that light can be either refracted or reflected by a material, depending on an angle with which the light impacts a surface of the material. Such is illustrated with respect to a material 50 in FIG. 4. Material 50 comprises an upper surface 52. Upper surface 52 is substantially planar. An axis "X" extends normal (i.e., perpendicular) to planar surface 52. A critical angle .theta. is defined as an angle relative to normal axis "X" wherein a beam of light impacting surface 52 passes from predominantly reflecting from surface 52 to predominantly refracting within surface 52. A critical angle is determined by the relative refractive indices of materials joining at a surface. Specifically, if light passes from a first material having a larger refractive to a second material with a lesser refractive index, a critical angle can be defined relative to an axis normal to a surface where the two materials meet. In the example of FIG. 4, such surface corresponds to surface 52. If light impacts surface 52 at an angle greater than angle .theta., the light will predominantly reflect from surface 52. Also, if light impacts surface 52 at an angle less than angle .theta., the light will predominantly pass into material 50 and refract within material 50. A critical angle .theta. for particular materials can be calculated from application of Snell's law and the relative amount of refraction and reflection can be determined. For a quartz/air interface a critical angle .theta. is about 40.4.degree., and for a quartz/water interface a critical angle .theta. is about 59.7.degree..
FIG. 4 also illustrates that a beam of light 55 can be directed into material 50 at an appropriate angle such that the light reflects from surfaces of material 50 to be contained internally of material 50. Such reflections are referred to as internal reflections. It is known that some of the light will actually extend slightly outward of a surface of material 50 (such as surface 52) as the light reflects internally from the surface. Such is illustrated by curved lines 57 in FIG. 4. Although the light extends slightly outward of the surfaces of material 50 as it is reflected within material 50, the light continues along the general path illustrated by beam 55. Accordingly, if material 50 is provided adjacent a sample, a light beam 55 can be provided to be internally reflective within material 50 and yet to stimulate molecules within the sample. Such use of internal reflections for stimulating molecules within a sample can be advantageous in situations wherein a sample is generally not transparent to a light source, such as, for example, when the sample is relatively turbid or optically dense. The amount by which light waves penetrate into a sample can be adjusted by changing a wavelength of the light, or by changing an angle at which the light internally reflects from surfaces of material 50.
In yet another aspect of the prior art, it is recognized that a sample's absorbance of light is directly proportional to a path length of light through the sample, and to a concentration of an absorbing species within the sample. Such relationship can be represented by the formula A=abc, wherein A is absorbance, a is a proportionality constant called absorptivity, b is a pathlength of light through the sample, and c is a concentration of absorbing species within the sample. Such relationship is referred to as Beer's Law. The Beer's Law relationship indicates that an amount of light absorbed is proportional to a concentration of an absorbing species. Another way of describing absorbance is as Log P.sub.0 /P, wherein P.sub.0 refers to the initial power of a light beam impacting a sample and P refers to the power of the beam exiting the sample. Most spectroscopic methods can detect and quantitate absorbing species only within a very narrow range of absorbance, such as, for example, a range of from about 0.05 to about 1.0. Accordingly, samples must be either diluted or concentrated to bring an absorbance of the sample within the appropriate range for the spectroscopic measurements. For samples that are extremely dilute, such as minor contaminants in sea water, it can be difficult and time consuming to adequately concentrate the samples for spectroscopic measurements. Accordingly, it would be desirable to develop spectroscopic methods that could be utilized over a wide range absorbance.
In contrast to spectroscopy methods which measure absorbance as Log P.sub.0 P, photoacoustic spectroscopy measures only P. This can provide enhanced sensitivity relative to other forms of spectroscopy in that it does not involve measuring a small signal "P" in the presence of a large background "P.sub.0 ". Also, an amplitude of a photoacoustic signal is believed to depend inversely on a volume of an excitation source (i.e., P/V.sub.0). In other words, Photoacoustic Theory predicts that an amplitude of a photoacoustic signal is proportional to an energy/volume ratio, wherein the energy is the energy generated by a measured transition and the volume is the volume of a sample. Photoacoustic spectroscopy can thus be advantageous over other forms of spectroscopy.